The invention relates to a method for the production of a component from steel by hot forming.
Such components are predominantly used in the automotive industry, but their use is also possible in mechanical engineering or civil engineering.
The hotly contested market forces automobile manufacturers to constantly seek solutions to reduce their fleet consumption while maintaining a highest possible comfort and occupant protection. Weight savings of all vehicle components play hereby a crucial role on one hand; on the other hand, a high passive occupant protection should be achieved which requires correspondingly high strengths statically/dynamically. In the event of a crash, it is also strived at a reduction of crash energy, this requires a ductile failure behavior.
Suppliers of source material attempt to take this necessity into account by reducing the wall thicknesses through use of high-strength or ultra high-strength steels, while improving component behavior during manufacture and during operation at the same time.
These steels must therefore satisfy comparatively high requirements in terms of strength, ductility, toughness, energy absorption, and corrosion resistance as well as handling capability, for example during cold forming and joining.
Against the background of the afore-mentioned aspects, the production of components of hot-malleable steels gains increasingly in importance because these components ideally meet the increased demands in terms of component properties while still requiring less material.
The production of components by quenching preliminary products of press hardenable steels by hot forming in a forming tool is known from DE 601 19 826 T2. A sheet metal blank, which has been heated beforehand to above the austenitizing temperature of 800-1200° C. and optionally provided with a metallic coating of zinc or zinc-based, is transformed here sometimes in a cooled tool through hot forming into a component, whereby the metal sheet or component undergoes in the forming tool during forming a hardening by quenching (press hardening) as a result of a rapid heat extraction and reaches the required strength properties due to the realized martensitic hardness microstructure.
The production of components by hot forming in a forming tool of quenched preliminary products of press hardenable steels and coated with an aluminum alloy is known from DE 699 33 751 T2. A sheet metal blank coated with an aluminum alloy is heated here before forming to above 700° C., with an intermetallically alloyed compound on the basis of iron, aluminum and silicon being realized on the surface, and the metal sheet is then formed and cooled down at a rate above the critical quenching rate.
The metallic coating is normally applied during the continuous hot-dip process on a hot or cold strip, e.g. by hot dip galvanizing or hot dip aluminizing at temperatures of about 460° C. (hot dip galvanizing) and about 680° C. (hot dip aluminizing).
Application of a metallic coating onto the workpiece (strip, blank) to be formed before hot forming is of advantage because the presence of the coating effectively prevents scaling of the base material.
Subsequently, the blank is cut to size for hot forming in the forming tool.
Known hot-formable steels for this application are e.g. the manganese-boron steel “22MnB5”.
The production of a component by press form hardening using known methods has several drawbacks.
In this method, the blank is heated to high temperatures above Ac3 so as to realize a complete austenitizing of the material and cooled after the pressing rapidly enough so as to establish a martensitic structure.
On the one hand, this method requires very much energy as a result of heating the preliminary product to austenitizing temperature and the transformation of ferrite to austenite, rendering the method expensive and producing significant amounts of CO2 and thus counteracting the demand for more energy-efficient methods.
When using sheet metal with a protective layer against scaling, extremely high demands are faced in terms of the temperature stability of the coating system, since the transformation at temperatures above the Ac3 temperature is generally significantly above 800° C. This has the consequence that the available process window is considerably smaller during press hardening compared to the use of material without a protection against scaling. For example, certain furnace times may not be exceeded. Furthermore, when the use of zinc-based coatings is involved, there is the risk of liquid metal embrittlement in these temperature ranges. Moreover, the high operating temperatures cause intense alloying of the metallic layer with iron, thus decreasing the corrosion protection effect in the finished component.
In addition to the described drawbacks, it should be noted that the known method is energy-intensive, resulting in high component prices and is CO2-intensive, causing excessive harm to the environment.
EP 1 783.234 A1 discloses a method for the production of products by forming at elevated temperatures, wherein a galvanized steel sheet is heated to a forming temperature of 450° C. to 700° C., then formed and slowly cooled down uncontrolled. In this way, the presence of excess stress should be avoided during hot forming. In general, it is stated that an improvement of the mechanical properties should be achieved in comparison to the cold forming.